US20020018434A1 - An Astigmatism Generating Device To Remove Comma Aberration And Spherical Aberration - Google Patents
An Astigmatism Generating Device To Remove Comma Aberration And Spherical Aberration Download PDFInfo
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- US20020018434A1 US20020018434A1 US09/085,229 US8522998A US2002018434A1 US 20020018434 A1 US20020018434 A1 US 20020018434A1 US 8522998 A US8522998 A US 8522998A US 2002018434 A1 US2002018434 A1 US 2002018434A1
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- light
- pickup device
- optical element
- optical pickup
- astigmatism
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
- G11B7/135—Means for guiding the beam from the source to the record carrier or from the record carrier to the detector
- G11B7/1365—Separate or integrated refractive elements, e.g. wave plates
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
- G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B7/0908—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for focusing only
- G11B7/0909—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for focusing only by astigmatic methods
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
- G11B7/135—Means for guiding the beam from the source to the record carrier or from the record carrier to the detector
- G11B7/1353—Diffractive elements, e.g. holograms or gratings
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
- G11B7/135—Means for guiding the beam from the source to the record carrier or from the record carrier to the detector
- G11B7/1381—Non-lens elements for altering the properties of the beam, e.g. knife edges, slits, filters or stops
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B2007/0003—Recording, reproducing or erasing systems characterised by the structure or type of the carrier
- G11B2007/0006—Recording, reproducing or erasing systems characterised by the structure or type of the carrier adapted for scanning different types of carrier, e.g. CD & DVD
Definitions
- the present invention relates to an optical pickup device for an optical recording and reproducing apparatus, such as an optical disc player.
- An optical disc player which can reproduce recording information from a recording media such as a laser disc (LD), a compact disc (CD) or a digital video (or versatile) disc (DVD), is well known.
- a so-called compatible disc player which can reproduce recording information from those different types of optical discs, is also known.
- an optical pickup device has an optical system for irradiating a light beam onto an optical disc and reading a return light from the optical disc.
- Japanese patent publication JP-B-2-8379 discloses an example of the optical pickup device utilizing a diffraction grating formed on a transparent parallel flat plate for beam deflection and guiding. As shown in FIG. 11, the optical pickup device has a light beam from a light source 1 focused on a pit train formed on an information recording surface 5 of an optical disc by an objective lens 4 .
- the four light-receiving elements of the photodetector 6 are divided and arranged parallel and perpendicular to the direction of the information track of the disc. Accordingly, when astigmatism is generated by the diffraction grating on the parallel flat plate 25 , the direction of an astigmatism axis coincides with a direction of dividing lines of the four light-receiving elements, and a focus error signal cannot be detected.
- the four light-receiving elements are inclined to a certain degree, a tracking error signal may leak into the focus error signal. Namely, when the return light is detected for controlling the position of the light beam relative to an information track on the information recording surface, an overlap of the detecting surface and the track direction, or a land border, causes the leak of the detection signal.
- the present invention is directed to an optical pickup device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
- An object of the present invention is to use a hologram optical element to improve the quality of the focus error signal.
- an optical pickup device including a photodetector divided into four light-receiving elements by two dividing lines, an incident optical system for irradiating a light beam onto an optical recording medium, and a receiving optical system for guiding a return light from the optical recording medium to the photodetector, wherein the receiving optical system includes astigmatism generating means for generating astigmatism having an astigmatism axis at an angle of 45 degree with respect to the two dividing lines.
- an optical pickup device including a light source for irradiating an incident light beam onto an optical recording medium, a photodetector having four light-receiving elements for receiving a return light beam from the optical recording medium, and a light deflecting device positioned in a forward light path between the light source and the optical recording medium and in a return light path between the optical recording medium and the photodetector, wherein the light deflecting device passes the incident light beam and deflects the return light towards the photodetector.
- FIG. 1 is a schematic perspective view of an optical system of an optical pickup device using a hologram optical element according to a first embodiment of the present invention
- FIG. 2 is a plan view of a grating pattern of the hologram optical element in the optical pickup device according to the present invention
- FIG. 3 is a flowchart explaining a wavefront design process of the hologram optical element in the optical pickup device according to the present invention
- FIG. 4 is a schematic view explaining a wavefront design of the hologram optical element in the optical pickup device according to the present invention.
- FIG. 5 is a schematic view explaining a wavefront design of the hologram optical element in the optical pickup device according to the present invention
- FIG. 6 is a schematic view explaining a wavefront design of the hologram optical element in the optical pickup device according to the present invention.
- FIGS. 7 A- 7 C are plan views of four light-receiving elements of a photodetector according to the present invention.
- FIG. 8 is an electrical diagram of the photodetector according to the embodiment of the present invention.
- FIG. 9 is a schematic perspective view of an optical system of an optical pickup device using a hologram optical element according to a second embodiment of the present invention.
- FIG. 10 is a view explaining the performance of a polarization hologram optical element in the optical pickup device of the present invention.
- FIG. 11 is a schematic perspective view of an optical system of an optical pickup device using a conventional diffraction grating.
- FIG. 1 is a schematic view of an optical pickup device in an optical recording and reproducing apparatus according to the present invention.
- the light beam from a TE mode laser diode 1 passes through a hologram optical element 2 (a deflecting device).
- the light beam is then focused onto a pit train formed on an information recording surface 5 of the optical disc by an objective lens 4 .
- the pit train runs in a y direction (a track direction).
- the laser diode 1 emits the light beam (a Gaussian beam) having a plane of vibration in the direction parallel to a junction interface and having an elliptic cross-sectional intensity distribution with a major longitudinal axis extending in a direction perpendicular to the junction interface of the laser diode 1 .
- an elliptical beam spot having the major longitudinal axis in the direction either parallel or perpendicular to a track direction is formed on the information recording surface 5 .
- the laser diode 1 is mounted on a base plate along with four light-receiving elements of a photodetector 6 .
- the reflected light from the information recording surface 5 again passes through the objective lens 4 and the hologram optical element 2 and is deflected from the forward light path.
- the light deflected by the hologram optical element 2 is focused onto the four light-receiving elements of the photodetector 6 .
- the hologram optical element 2 that generates the astigmatism is employed as the deflecting device.
- FIG. 2 is a plan view of a grating pattern of the hologram optical element 2 as seen from the z direction.
- the hologram optical element 2 is a parallel flat plate of a transparent material having a diffraction relief formed on its main surface, generating the astigmatism.
- the hologram optical element 2 is designed based on the interference of the light beam from the laser diode 1 and the reflected light at the four light-receiving elements. It may be designed by a computer method shown in FIG. 3 using any number of well-known techniques.
- the wavefront for the grating pattern is determined by a high refractive index method or a light following method using a phase function method.
- a parallel flat plate 70 (refractive index n) having a thickness tl is positioned in a light path of a light diverging from a point A (wavelength ⁇ 1 ), which corresponds to the laser diode.
- Initial values of paraneters i.e., the coordinates of the point A, ⁇ 1 , t 1 and n, are properly determined.
- the wavefront at the coordinates of point B including spherical aberration is calculated. The result of the calculation is stored.
- the spherical aberration in the light caused by the parallel flat plate 70 is removed by a correction in step S 2 of FIG. 3.
- the amount of the astigmatism generated by the parallel flat plate 70 is adjusted by changing the thickness tl.
- step S 2 the stored wavefront at the point B is converged,
- the return light passes through the parallel flat plate 70 having the thickness of tl (not shown) and is focused onto the point A.
- Two parallel flat plates 71 each having a thickness t 2 and a refractive index n are positioned in the light path of the return light (instead of the parallel flat plate 70 of FIG. 4).
- the parallel flat plates 71 are positioned apart from each other and inclined at an angle of ⁇ and ⁇ with respect to a plane perpendicular to a light axis in order to be a mirror image with respect to the plane.
- the wavefront at a point C after passing through the parallel flat plates 71 is calculated. Parameters such as the coordinates of the point C, t 2 , ⁇ and ⁇ are properly selected.
- the wavefront passing through the two parallel flat plates 71 has astigmatism and spherical aberration, but no coma aberration.
- the spherical aberration can be adjusted by changing the thickness t 2 of the parallel flat plates 71 and therefore the spherical aberration caused in step SI can be removed.
- the wavefront at the point C after passing through the parallel flat plates 71 and having a certain amount of astigmatism, but no coma or spherical aberration is calculated and stored.
- step S 6 the stored wavefront at a point C is again diverged.
- the wavefront at a point H that is inclined at a certain angle (angle a) from the light axis is then calculated.
- the point to which the light is converged from the stored wavefront at the point C corresponds to the position of the four light-receiving elements.
- the grating pattern of the hologram optical element 2 can be designed based on the interference of the calculated wavefront and the wavefront of the light diverging from a point O(the position of the laser diode 1 ). Parameters such as the coordinates of the point H and the point O and the angle ⁇ are properly selected.
- the obtained interference pattern at the point H is then stored and employed as the grating pattern of the hologram optical element 2 .
- the hologram optical element 2 shown in FIG. 2 By forming the grating pattern obtained as described above on a transparent substrate, the hologram optical element 2 shown in FIG. 2 generates astigmatism without causing coma or spherical aberration and functions as a lens for changing the focal length of the light beam.
- the hologram optical element 2 has a diffraction relief that is so designed that the diffracted light forms a light spot near the center of the four light-receiving elements of the photodetector 6 .
- a circular light spot is formed on the four light-receiving elements as shown in FIG. 7A.
- an elliptic light spot is formed on the four light-receiving elements in the direction of diagonal line of the elements as shown in FIGS. 7B and 7C. Namely, the hologram optical element 2 generates astigmatism.
- the photodetector 6 has the four light-receiving elements divided by two lines L 1 and L 2 crossing each other at right angles.
- the light spot irradiated onto each of four light-receiving elements is converted into electric signals (through photoelectric conversion) and provided to a focus error detecting circuit 12 , as shown in FIG. 8.
- the focus error detecting circuit 12 produces a focus error signal (FES) based on a signal from the photodetector 6 and provides the FES to an actuator drive circuit (not shown).
- the actuator drive circuit supplies a focusing drive signal to an actuator (not shown).
- the actuator moves the objective lens in the direction of the optical axis in accordance with the focusing drive signal.
- the focus error detecting circuit 12 is connected to the photodetector 6 .
- the photodetector 6 is divided into four light-receiving elements DET 1 to DET 4 by the two dividing lines L 1 and L 2 crossing each other at right angles.
- the four elements DET 1 to DET 4 correspond to first through fourth quadrants that are mutually independent.
- the photodetector 6 is positioned so that one division line (L 1 or L 2 ) is parallel to the track direction and the other division line is parallel to the radial direction of the optical disc.
- the photoelectric conversion outputs from the elements DET 1 and DET 3 which are located symmetrically with respect to the center O of the light-receiving surface, are added by an adder 22 .
- the photoelectric conversion outputs from the elements DET 2 and DET 4 are added by an adder 21 .
- the output signals from the adder 21 and 22 are fed into a differential amplifier 23 .
- the differential amplifier 23 calculates a difference between the output signals and outputs the differential signal as the focus error signal FES.
- the focus error detection circuit 12 produces the focus error components by adding the output of the four light-receiving elements of the photodetector 6 using the adders 21 and 22 and calculating the differential signal of the outputs thereof using the differential amplifier 23 .
- the intensity distribution is symmetrical with respect to the center O, i.e., symmetrical with respect to the track direction and the radial direction, and the circular light spot shown in FIG. 7A is formed on the photodetector 6 .
- values obtained by adding the photoelectric conversion outputs of the elements existing on the diagonal lines are equal to each other and the focus error is 0 .
- the amount of the return light at a portion corresponding to a shadow of the track fluctuates on the four light-receiving elements of the photodetector 6 .
- the focus error signal is obtained from the difference between outputs of the diagonal light-receiving elements.
- the tracking error signal influences the focus error signal. Therefore, it is necessary for the extending direction of the shadow of the pit train (track) to coincide with the direction of the dividing line (L 1 or L 2 ) of the four light-receiving elements.
- the four light-receiving elements in FIG. 1 must have the dividing lines L 1 and L 2 coincide with the directions of the x axis and the y axis, respectively.
- This arrangement also coincides with the direction of the division when the tracking servo is controlled by phase difference (or time difference) methods. Accordingly, the directions of the dividing lines of the four light-receiving elements are determined depending on the direction of the pit train (track) on the information recording surface.
- the astigmatism method is employed for the focus servo control, the direction of the astigmatism axis is restricted in accordance with the direction of the light-receiving elements.
- the direction of the astigmatism generated for the focus servo control is limited to the direction at an angle of 45 degrees in the x-y plane shown in FIG. 1.
- the TE mode laser diode is used.
- a TM mode laser diode may similarly be used.
- the light intensity distribution on the information recording surface extends in the radial direction even if the plane of vibration of the incident beam is in the y direction, and the direction of astigmatism is similarly limited to a direction of 45 degrees in the x-y plane.
- an optical pickup irradiates the light beam on the optical recording medium via a 1 ⁇ 4 wavelength plate and an objective lens.
- the optical pickup employs a transparent flat polarization hologram optical element formed of a uniaxial crystal having a diffraction relief for generating astigmatism.
- FIG. 9 is a schematic diagram of the optical pickup using the polarization hologram optical element in the optical recording and reproducing apparatus.
- a light beam from a laser diode 1 is focused onto a pit train on an information recording surface 5 of an optical disc by an objective lens 4 via a polarization hologram optical element 2 (a deflecting device) and a 1 ⁇ 4 wavelength plate 3 .
- the incident light beam has a plane of vibration in the x direction, which is parallel to a junction interface and an emitting surface (cleavage plane) of the laser diode 1 .
- the laser diode 1 is arranged so that the plane of vibration exists in the x direction.
- the incident light beam has an elliptic Gaussian distribution with a minor axis oriented in the x direction and a major longitudinal axis oriented in a y direction. As shown in FIGS. 9 and 10, the incident light beam passes through the polarization hologram optical element 2 and is converted from linear polarization into circular polarization by the 1 ⁇ 4 wavelength plate 3 . The light beam is then focused onto the information recording surface 5 by the objective lens 4 .
- the light beam having the circular polarization has been reflected and diffracted by the information recording surface 5 and again passes through the objective lens 4 .
- the light beam of the circular polarization is then converted by the 114 wavelength plate 3 into a linear polarization beam having a phase difference of 90 degrees with respect to the incident light beam.
- the plane of vibration is also rotated to the y direction.
- the light beam having the linear polarization is diffracted by the polarization hologram optical element 2 and is thus separated from the incident light path.
- the reflected light from the information recording surface 5 is deflected by the polarization hologram optical element 2 via the objective lens 4 and the 1 ⁇ 4 wavelength plate 3 by which the plane of vibration is inclined at an angle of 90 degrees from the incident light beam.
- the deflected light beam is then focused onto the four light-receiving elements of the photodetector 6 .
- the incident light beam passes through the polarization hologram optical element 2 , which produces astigmatism when the plane of vibration is oriented in the x direction.
- the incident light beam is diffracted by the polarization hologram optical element 2 .
- the polarization hologram optical element 2 in the second embodiment includes a first transparent portion 11 formed of a transparent uniaxial crystal and a second transparent portion 12 having a refractive index substantially identical to an ordinary index n o or an extraordinary index n, of the uniaxial crystal.
- the first and the second transparent portions II and 12 are joined via a composition surface 13 having a diffraction relief formed thereon.
- the polarization hologram optical element 2 is a flat plate having parallel surfaces on both sides.
- the diffraction grating pattern for generating the astigmatism is formed on the composition surface 13 by the method described in the description of the first embodiment.
- the angle of the optical crystal axis with respect to the optical axis of the incident light, and the ordinary index n o of the uniaxial crystal for the first transparent portion 11 are properly selected.
- the material of the second transparent portion 12 is also selected to have the same refractive index as the ordinary index n u , of the uniaxial crystal.
- the polarization hologram optical element 2 performs different functions depending on the polarization of the incident light beam.
- the first transparent portion 11 of the polarization hologram optical element 2 is formed of a negative (i e., n o >n c ) uniaxial optical crystal.
- the second transparent portion 12 is formed of the same material as the first transparent portion 11 , having the crystal axis in the direction of the optical axis and joined to the first transparent portion 11 via the composition surface 13 .
- the optical axis of the first transparent portion 11 is not parallel to the optical axis of the incident light, but is perpendicular to the plane of the paper, for example, the light beam having the plane of vibration parallel to the plane of the paper is an ordinary ray.
- the refractive index of the first transparent portion 11 is n o . Since the refractive index of the second transparent portion 12 is also n o , the polarization hologram optical element 2 functions as a transparent parallel flat plate having the refractive index n, as a whole.
- the return light has its plane of vibration perpendicular to the plane of the paper because the light passes through the 114 wavelength plate twice.
- the return light is the ordinary ray, and therefore the refractive index is n o , same as the incident light.
- the first transparent portion 11 the light behaves as the extraordinary ray, and the refractive index is n e .
- the polarization hologram optical element 2 functions as a diffraction grating in which the diffraction relief is formed as a borderline.
- the polarization hologram optical element 2 the parallel flat plate
- the return light (the arrow pointing left) reflected from the optical disc passes through the polarization hologram optical element 2 and is detected by the photodetector 6 .
- the first and second transparent portions 11 and 12 are joined so that their optical crystal axes cross at right angles.
- the difference in the refractive index between the first and second transparent portions 11 and 12 can be maximized.
- the deflecting angle of the return light can be adjusted by changing the angle of the composition surface 13 with respect to the optical axis, which is perpendicular to the optical axis in the second embodiment.
- the refractive index of the second transparent portion 12 is substantially identical to the ordinary index n, of the first transparent portion 11 .
- the polarization hologram optical element 2 functions as a parallel flat plate and a hologram for the incident light and the return light, respectively.
- the first and the second transparent portions 11 and 12 may be formed of different uniaxial crystal materials selected from among a variety of materials. Furthermore, in addition to the negative uniaxial crystal, a positive uniaxial material may also be used.
- Both of the first and the second transparent portions 11 and 12 are not necessarily formed of anisotropic materials. Namely, it is also permissible to form the first transparent portion 11 of a uniaxial crystal and the second transparent portion of an isotropic material, and vice versa
- the second transparent portion 12 may be formed of an isotropic material such as the optical glass having a refractive index n g equal to the ordinary index no of the first transparent portion 11 of the negative uniaxial crystal.
- the polarization hologram optical element 2 functions as a diffraction grating because n e ⁇ n g .
- the polarization hologram optical element 2 may be formed as a single flat plate of a transparent uniaxial crystal without combining two portions.
- the polarization hologram optical element may also be formed as a single flat plate having a diffraction relief for generating astigmatism on at least one main surface and having a uniaxial crystal filled in a concave portion of the diffraction relief.
- the return light is separated from the incident light path by the hologram optical element 2 .
- other deflecting devices such as a polarization beam splitter, may also be used instead of the hologram optical element 2 .
- the return light from the optical disc is reflected by the polarization beam splitter at a right angle in the x direction and advances toward the photodetector 6 that is positioned so that the four light-receiving elements are located in the y-z plane in FIG. 1.
- the direction of the dividing lines of the light-receiving elements and the direction of the astigmatism axis are at an angle of 45 degrees with respect to the y axis or the z axis.
- an astigmatism generating device such as a convex cylindrical lens is inserted for generating astigmatism having an axis at an angle of 45 degrees.
- the optical pickup of a finite conjugate type system is described.
- the optical pickup of an infinite conjugate type system may also be used in which the objective lens, the hologram optical element 2 , and the 1 ⁇ 4 wavelength plate can be controlled as an integral member and a collimating lens is inserted in front of the light source.
- the efficiency of the light being utilized is improved and the number of optical parts can be reduced.
- the efficiency of the light being utilized can be further improved by using a polarization element as the deflecting device.
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Abstract
Description
- This application claims the benefit of Japanese Application No. 9-138779, filed in Japan on May 28, 1997, which is hereby incorporated by reference.
- 1. Field of the Invention
- The present invention relates to an optical pickup device for an optical recording and reproducing apparatus, such as an optical disc player.
- 2. Discussion of the Related Art
- An optical disc player, which can reproduce recording information from a recording media such as a laser disc (LD), a compact disc (CD) or a digital video (or versatile) disc (DVD), is well known. A so-called compatible disc player, which can reproduce recording information from those different types of optical discs, is also known.
- In such a disc player, an optical pickup device has an optical system for irradiating a light beam onto an optical disc and reading a return light from the optical disc. Japanese patent publication JP-B-2-8379 discloses an example of the optical pickup device utilizing a diffraction grating formed on a transparent parallel flat plate for beam deflection and guiding. As shown in FIG. 11, the optical pickup device has a light beam from a
light source 1 focused on a pit train formed on aninformation recording surface 5 of an optical disc by anobjective lens 4. The return light reflected from theinformation recording surface 5 again passes through theobjective lens 4, is deflected by a diffraction grating formed on a parallelflat plate 25, and is guided and focused onto four light-receiving elements of aphotodetector 6. - In the optical pickup device described above, the four light-receiving elements of the
photodetector 6 are divided and arranged parallel and perpendicular to the direction of the information track of the disc. Accordingly, when astigmatism is generated by the diffraction grating on the parallelflat plate 25, the direction of an astigmatism axis coincides with a direction of dividing lines of the four light-receiving elements, and a focus error signal cannot be detected. When the four light-receiving elements are inclined to a certain degree, a tracking error signal may leak into the focus error signal. Namely, when the return light is detected for controlling the position of the light beam relative to an information track on the information recording surface, an overlap of the detecting surface and the track direction, or a land border, causes the leak of the detection signal. - Accordingly, the present invention is directed to an optical pickup device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
- An object of the present invention is to use a hologram optical element to improve the quality of the focus error signal.
- Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
- To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, in one aspect of the present invention there is provided an optical pickup device including a photodetector divided into four light-receiving elements by two dividing lines, an incident optical system for irradiating a light beam onto an optical recording medium, and a receiving optical system for guiding a return light from the optical recording medium to the photodetector, wherein the receiving optical system includes astigmatism generating means for generating astigmatism having an astigmatism axis at an angle of 45 degree with respect to the two dividing lines.
- In another aspect of the present invention there is provided an optical pickup device including a light source for irradiating an incident light beam onto an optical recording medium, a photodetector having four light-receiving elements for receiving a return light beam from the optical recording medium, and a light deflecting device positioned in a forward light path between the light source and the optical recording medium and in a return light path between the optical recording medium and the photodetector, wherein the light deflecting device passes the incident light beam and deflects the return light towards the photodetector.
- It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
- The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.
- In the drawings:
- FIG. 1 is a schematic perspective view of an optical system of an optical pickup device using a hologram optical element according to a first embodiment of the present invention;
- FIG. 2 is a plan view of a grating pattern of the hologram optical element in the optical pickup device according to the present invention;
- FIG. 3 is a flowchart explaining a wavefront design process of the hologram optical element in the optical pickup device according to the present invention;
- FIG. 4 is a schematic view explaining a wavefront design of the hologram optical element in the optical pickup device according to the present invention;
- FIG. 5 is a schematic view explaining a wavefront design of the hologram optical element in the optical pickup device according to the present invention;
- FIG. 6 is a schematic view explaining a wavefront design of the hologram optical element in the optical pickup device according to the present invention;
- FIGS.7A-7C are plan views of four light-receiving elements of a photodetector according to the present invention;
- FIG. 8 is an electrical diagram of the photodetector according to the embodiment of the present invention;
- FIG. 9 is a schematic perspective view of an optical system of an optical pickup device using a hologram optical element according to a second embodiment of the present invention;
- FIG. 10 is a view explaining the performance of a polarization hologram optical element in the optical pickup device of the present invention; and
- FIG. 11 is a schematic perspective view of an optical system of an optical pickup device using a conventional diffraction grating.
- Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.
- FIG. 1 is a schematic view of an optical pickup device in an optical recording and reproducing apparatus according to the present invention.
- In an optical system of a forward light path in which a light beam is incident onto an optical disc, the light beam from a TE mode laser diode1 (a light source) passes through a hologram optical element 2 (a deflecting device). The light beam is then focused onto a pit train formed on an
information recording surface 5 of the optical disc by anobjective lens 4. The pit train runs in a y direction (a track direction). Thelaser diode 1 emits the light beam (a Gaussian beam) having a plane of vibration in the direction parallel to a junction interface and having an elliptic cross-sectional intensity distribution with a major longitudinal axis extending in a direction perpendicular to the junction interface of thelaser diode 1. Depending on a position of thelaser diode 1, an elliptical beam spot having the major longitudinal axis in the direction either parallel or perpendicular to a track direction is formed on theinformation recording surface 5. Thelaser diode 1 is mounted on a base plate along with four light-receiving elements of aphotodetector 6. - In a receiving optical system of the return light path (which is the same as the optical system of the incident beam), the reflected light from the
information recording surface 5 again passes through theobjective lens 4 and the hologramoptical element 2 and is deflected from the forward light path. The light deflected by the hologramoptical element 2 is focused onto the four light-receiving elements of thephotodetector 6. - In the first embodiment, the hologram
optical element 2 that generates the astigmatism is employed as the deflecting device. FIG. 2 is a plan view of a grating pattern of the hologramoptical element 2 as seen from the z direction. The hologramoptical element 2 is a parallel flat plate of a transparent material having a diffraction relief formed on its main surface, generating the astigmatism. - The hologram
optical element 2 is designed based on the interference of the light beam from thelaser diode 1 and the reflected light at the four light-receiving elements. It may be designed by a computer method shown in FIG. 3 using any number of well-known techniques. The wavefront for the grating pattern is determined by a high refractive index method or a light following method using a phase function method. - In step S1, of FIG. 3 and schematically shown in FIG. 4, a parallel flat plate 70 (refractive index n) having a thickness tl is positioned in a light path of a light diverging from a point A (wavelength λ1), which corresponds to the laser diode. Initial values of paraneters, i.e., the coordinates of the point A, λ1, t1 and n, are properly determined. With regard to the light after passing through the parallel
flat plate 70, the wavefront at the coordinates of point B including spherical aberration is calculated. The result of the calculation is stored. The spherical aberration in the light caused by the parallelflat plate 70 is removed by a correction in step S2 of FIG. 3. The amount of the astigmatism generated by the parallelflat plate 70 is adjusted by changing the thickness tl. - In step S2, as shown in FIG. 5, the stored wavefront at the point B is converged, The return light passes through the parallel
flat plate 70 having the thickness of tl (not shown) and is focused onto the point A. Two parallelflat plates 71 each having a thickness t2 and a refractive index n are positioned in the light path of the return light (instead of the parallelflat plate 70 of FIG. 4). The parallelflat plates 71 are positioned apart from each other and inclined at an angle of θ and −θ with respect to a plane perpendicular to a light axis in order to be a mirror image with respect to the plane. The wavefront at a point C after passing through the parallelflat plates 71 is calculated. Parameters such as the coordinates of the point C, t2, θ and −θ are properly selected. - As the light is converged from a point B, the wavefront passing through the two parallel
flat plates 71 has astigmatism and spherical aberration, but no coma aberration. The spherical aberration can be adjusted by changing the thickness t2 of the parallelflat plates 71 and therefore the spherical aberration caused in step SI can be removed. Thus, the wavefront at the point C after passing through the parallelflat plates 71 and having a certain amount of astigmatism, but no coma or spherical aberration, is calculated and stored. - In step S6, as shown in FIG. 6, the stored wavefront at a point C is again diverged. The wavefront at a point H that is inclined at a certain angle (angle a) from the light axis is then calculated. Here, the point to which the light is converged from the stored wavefront at the point C corresponds to the position of the four light-receiving elements. At the point H (the position of the hologram optical element 2), the grating pattern of the hologram
optical element 2 can be designed based on the interference of the calculated wavefront and the wavefront of the light diverging from a point O(the position of the laser diode 1). Parameters such as the coordinates of the point H and the point O and the angle α are properly selected. The obtained interference pattern at the point H is then stored and employed as the grating pattern of the hologramoptical element 2. - By forming the grating pattern obtained as described above on a transparent substrate, the hologram
optical element 2 shown in FIG. 2 generates astigmatism without causing coma or spherical aberration and functions as a lens for changing the focal length of the light beam. - The hologram
optical element 2 has a diffraction relief that is so designed that the diffracted light forms a light spot near the center of the four light-receiving elements of thephotodetector 6. When the light beam is in focus on the information recording surface of the disc, a circular light spot is formed on the four light-receiving elements as shown in FIG. 7A. When the light beam is out of focus, an elliptic light spot is formed on the four light-receiving elements in the direction of diagonal line of the elements as shown in FIGS. 7B and 7C. Namely, the hologramoptical element 2 generates astigmatism. - The
photodetector 6 has the four light-receiving elements divided by two lines L1 and L2 crossing each other at right angles. The light spot irradiated onto each of four light-receiving elements is converted into electric signals (through photoelectric conversion) and provided to a focus error detecting circuit 12, as shown in FIG. 8. The focus error detecting circuit 12 produces a focus error signal (FES) based on a signal from thephotodetector 6 and provides the FES to an actuator drive circuit (not shown). The actuator drive circuit supplies a focusing drive signal to an actuator (not shown). The actuator moves the objective lens in the direction of the optical axis in accordance with the focusing drive signal. - As shown in FIG. 8, the focus error detecting circuit12 is connected to the
photodetector 6. Thephotodetector 6 is divided into four light-receiving elements DET1 to DET4 by the two dividing lines L1 and L2 crossing each other at right angles. The four elements DET1 to DET4 correspond to first through fourth quadrants that are mutually independent. Thephotodetector 6 is positioned so that one division line (L1 or L2) is parallel to the track direction and the other division line is parallel to the radial direction of the optical disc. The photoelectric conversion outputs from the elements DET1 and DET3, which are located symmetrically with respect to the center O of the light-receiving surface, are added by anadder 22. Similarly, the photoelectric conversion outputs from the elements DET2 and DET4 are added by an adder 21. The output signals from theadder 21 and 22 are fed into a differential amplifier 23. The differential amplifier 23 calculates a difference between the output signals and outputs the differential signal as the focus error signal FES. - As explained above, the focus error detection circuit12 produces the focus error components by adding the output of the four light-receiving elements of the
photodetector 6 using theadders 21 and 22 and calculating the differential signal of the outputs thereof using the differential amplifier 23. When the light beam is in focus, the intensity distribution is symmetrical with respect to the center O, i.e., symmetrical with respect to the track direction and the radial direction, and the circular light spot shown in FIG. 7A is formed on thephotodetector 6. In this case, values obtained by adding the photoelectric conversion outputs of the elements existing on the diagonal lines are equal to each other and the focus error is 0. When the light beam is out of focus, the elliptic light spot having an axis in the diagonal direction is formed on thephotodetector 6, as shown in FIGS. 7B and 7C. Accordingly, values obtained by adding the photoelectric conversion outputs of the diagonal pairs of the light-receiving elements are different from each other. - In the optical system described above, when the light beam traverses the pit train (track) due to an erroneous tracking operation, the amount of the return light at a portion corresponding to a shadow of the track fluctuates on the four light-receiving elements of the
photodetector 6. In the astigmatism focusing method, the focus error signal is obtained from the difference between outputs of the diagonal light-receiving elements. Thus, if the shadow of the track is projected onto the diagonal pairs of light-receiving elements, the tracking error signal influences the focus error signal. Therefore, it is necessary for the extending direction of the shadow of the pit train (track) to coincide with the direction of the dividing line (L1 or L2) of the four light-receiving elements. For the above reason, the four light-receiving elements in FIG. 1 must have the dividing lines L1 and L2 coincide with the directions of the x axis and the y axis, respectively. This arrangement also coincides with the direction of the division when the tracking servo is controlled by phase difference (or time difference) methods. Accordingly, the directions of the dividing lines of the four light-receiving elements are determined depending on the direction of the pit train (track) on the information recording surface. Thus, when the light path of the incident light and that of the return light are separated using the deflecting device, and the astigmatism method is employed for the focus servo control, the direction of the astigmatism axis is restricted in accordance with the direction of the light-receiving elements. - In the astigmatism method using the four light-receiving elements, as shown in FIG. 1, the direction of the astigmatism generated for the focus servo control is limited to the direction at an angle of 45 degrees in the x-y plane shown in FIG. 1.
- In the embodiment described above, the TE mode laser diode is used. However, a TM mode laser diode may similarly be used. With the TM mode laser diode, the light intensity distribution on the information recording surface extends in the radial direction even if the plane of vibration of the incident beam is in the y direction, and the direction of astigmatism is similarly limited to a direction of 45 degrees in the x-y plane.
- A second embodiment of the present invention will now be explained. In the second embodiment, an optical pickup irradiates the light beam on the optical recording medium via a ¼ wavelength plate and an objective lens. The optical pickup employs a transparent flat polarization hologram optical element formed of a uniaxial crystal having a diffraction relief for generating astigmatism. With this, the structure of the compatible disc player for the DVD and DVD-RAM can be simplified, miniaturized, and its cost can be reduced.
- FIG. 9 is a schematic diagram of the optical pickup using the polarization hologram optical element in the optical recording and reproducing apparatus. In the light path of the incident beam, a light beam from a
laser diode 1 is focused onto a pit train on aninformation recording surface 5 of an optical disc by anobjective lens 4 via a polarization hologram optical element 2 (a deflecting device) and a ¼wavelength plate 3. The incident light beam has a plane of vibration in the x direction, which is parallel to a junction interface and an emitting surface (cleavage plane) of thelaser diode 1. Namely, thelaser diode 1 is arranged so that the plane of vibration exists in the x direction. The incident light beam has an elliptic Gaussian distribution with a minor axis oriented in the x direction and a major longitudinal axis oriented in a y direction. As shown in FIGS. 9 and 10, the incident light beam passes through the polarization hologramoptical element 2 and is converted from linear polarization into circular polarization by the ¼wavelength plate 3. The light beam is then focused onto theinformation recording surface 5 by theobjective lens 4. - In a receiving optical system of a return light path, the light beam having the circular polarization has been reflected and diffracted by the
information recording surface 5 and again passes through theobjective lens 4. The light beam of the circular polarization is then converted by the 114wavelength plate 3 into a linear polarization beam having a phase difference of 90 degrees with respect to the incident light beam. The plane of vibration is also rotated to the y direction. The light beam having the linear polarization is diffracted by the polarization hologramoptical element 2 and is thus separated from the incident light path. As mentioned above, the reflected light from theinformation recording surface 5 is deflected by the polarization hologramoptical element 2 via theobjective lens 4 and the ¼wavelength plate 3 by which the plane of vibration is inclined at an angle of 90 degrees from the incident light beam. The deflected light beam is then focused onto the four light-receiving elements of thephotodetector 6. As shown in FIGS. 9 and 10, the incident light beam passes through the polarization hologramoptical element 2, which produces astigmatism when the plane of vibration is oriented in the x direction. When the plane of vibration is oriented in the y direction, the incident light beam is diffracted by the polarization hologramoptical element 2. - As shown in FIG. 10, the polarization hologram
optical element 2 in the second embodiment includes a first transparent portion 11 formed of a transparent uniaxial crystal and a second transparent portion 12 having a refractive index substantially identical to an ordinary index no or an extraordinary index n, of the uniaxial crystal. The first and the second transparent portions II and 12 are joined via acomposition surface 13 having a diffraction relief formed thereon. The polarization hologramoptical element 2 is a flat plate having parallel surfaces on both sides. The diffraction grating pattern for generating the astigmatism is formed on thecomposition surface 13 by the method described in the description of the first embodiment. - The angle of the optical crystal axis with respect to the optical axis of the incident light, and the ordinary index no of the uniaxial crystal for the first transparent portion 11 are properly selected. The material of the second transparent portion 12 is also selected to have the same refractive index as the ordinary index nu, of the uniaxial crystal. Thus, the polarization hologram
optical element 2 performs different functions depending on the polarization of the incident light beam. - As shown in FIG. 10, for example, the first transparent portion11 of the polarization hologram
optical element 2 is formed of a negative (i e., no>nc) uniaxial optical crystal. The second transparent portion 12 is formed of the same material as the first transparent portion 11, having the crystal axis in the direction of the optical axis and joined to the first transparent portion 11 via thecomposition surface 13. When the optical axis of the first transparent portion 11 is not parallel to the optical axis of the incident light, but is perpendicular to the plane of the paper, for example, the light beam having the plane of vibration parallel to the plane of the paper is an ordinary ray. The refractive index of the first transparent portion 11 is no. Since the refractive index of the second transparent portion 12 is also no, the polarization hologramoptical element 2 functions as a transparent parallel flat plate having the refractive index n, as a whole. - The return light has its plane of vibration perpendicular to the plane of the paper because the light passes through the 114 wavelength plate twice. In the second transparent portion12, the return light is the ordinary ray, and therefore the refractive index is no, same as the incident light. In the first transparent portion 11, however, the light behaves as the extraordinary ray, and the refractive index is ne. Accordingly, for the return light, the polarization hologram
optical element 2 functions as a diffraction grating in which the diffraction relief is formed as a borderline. - When the light beam (the arrow pointing right) having the plane of vibration parallel to the plane of the paper is incident on the optical disc via the polarization hologram optical element2 (the parallel flat plate), the return light (the arrow pointing left) reflected from the optical disc passes through the polarization hologram
optical element 2 and is detected by thephotodetector 6. In the polarization hologramoptical element 2, the first and second transparent portions 11 and 12 are joined so that their optical crystal axes cross at right angles. Thus, the difference in the refractive index between the first and second transparent portions 11 and 12 can be maximized. Furthermore, the deflecting angle of the return light can be adjusted by changing the angle of thecomposition surface 13 with respect to the optical axis, which is perpendicular to the optical axis in the second embodiment. - In the second embodiment, the refractive index of the second transparent portion12 is substantially identical to the ordinary index n, of the first transparent portion 11. However, even if the refractive index of the second transparent portion 12 is substantially identical to the extraordinary index ne of the first transparent portion 11, the polarization hologram
optical element 2 functions as a parallel flat plate and a hologram for the incident light and the return light, respectively. - In the second embodiment, the first and the second transparent portions11 and 12 may be formed of different uniaxial crystal materials selected from among a variety of materials. Furthermore, in addition to the negative uniaxial crystal, a positive uniaxial material may also be used.
- Both of the first and the second transparent portions11 and 12 are not necessarily formed of anisotropic materials. Namely, it is also permissible to form the first transparent portion 11 of a uniaxial crystal and the second transparent portion of an isotropic material, and vice versa
- The second transparent portion12 may be formed of an isotropic material such as the optical glass having a refractive index ng equal to the ordinary index no of the first transparent portion 11 of the negative uniaxial crystal. The polarization hologram
optical element 2 with such a structure functions as a transparent parallel flat plate having the refractive index no (because no=ng) for the incident light beam. For the return light, on the other hand, the polarization hologramoptical element 2 functions as a diffraction grating because ne≢ng. - The polarization hologram
optical element 2 may be formed as a single flat plate of a transparent uniaxial crystal without combining two portions. The polarization hologram optical element may also be formed as a single flat plate having a diffraction relief for generating astigmatism on at least one main surface and having a uniaxial crystal filled in a concave portion of the diffraction relief. - In the embodiments described above, the return light is separated from the incident light path by the hologram
optical element 2. However, other deflecting devices, such as a polarization beam splitter, may also be used instead of the hologramoptical element 2. With such a structure, the return light from the optical disc is reflected by the polarization beam splitter at a right angle in the x direction and advances toward thephotodetector 6 that is positioned so that the four light-receiving elements are located in the y-z plane in FIG. 1. The direction of the dividing lines of the light-receiving elements and the direction of the astigmatism axis are at an angle of 45 degrees with respect to the y axis or the z axis. Between the polarization beam splitter and thephotodetector 6, an astigmatism generating device such as a convex cylindrical lens is inserted for generating astigmatism having an axis at an angle of 45 degrees. - In the embodiments described above, the optical pickup of a finite conjugate type system is described. However, the optical pickup of an infinite conjugate type system may also be used in which the objective lens, the hologram
optical element 2, and the ¼ wavelength plate can be controlled as an integral member and a collimating lens is inserted in front of the light source. - According to the present invention, because a deflecting device that generates astigmatism is employed, the efficiency of the light being utilized is improved and the number of optical parts can be reduced. The efficiency of the light being utilized can be further improved by using a polarization element as the deflecting device.
- It will be apparent to those skilled in the art that various modifications and variations can be made in the optical pickup device of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
Claims (18)
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
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JP9-138779 | 1997-05-28 | ||
JP9138779A JPH10333025A (en) | 1997-05-28 | 1997-05-28 | Optical pickup device |
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US20020018434A1 true US20020018434A1 (en) | 2002-02-14 |
US6445668B2 US6445668B2 (en) | 2002-09-03 |
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US09/085,229 Expired - Fee Related US6445668B2 (en) | 1997-05-28 | 1998-05-27 | Astigmatism generating device to remove comma aberration and spherical aberration |
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US (1) | US6445668B2 (en) |
JP (1) | JPH10333025A (en) |
Cited By (1)
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US20080137495A1 (en) * | 2005-03-08 | 2008-06-12 | Mitsubishi Electric Corporation | Optical Device and Optical Disc Apparatus Utilizing the Optical Device |
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US6967908B2 (en) * | 2000-09-07 | 2005-11-22 | Pioneer Corporation | Optical pickup device with focus error detecting optical element and method for focus error detection |
JP2002358690A (en) * | 2001-05-30 | 2002-12-13 | Pioneer Electronic Corp | Optical read-out device with aberration correction function |
JP2003045042A (en) * | 2001-07-31 | 2003-02-14 | Toshiba Corp | Thickness irregularity correction method for information recording medium and information recording and reproducing device using thickness irregularity correction method |
JP2011008852A (en) * | 2009-06-25 | 2011-01-13 | Sanyo Electric Co Ltd | Optical pickup device |
JP2012084200A (en) * | 2010-10-12 | 2012-04-26 | Hitachi Media Electoronics Co Ltd | Optical pickup and optical information recording and reproducing device |
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US5804814A (en) * | 1994-05-20 | 1998-09-08 | Musha; Toru | Optical pick-up head and integrated type optical unit for use in optical pick-up head |
NL7907216A (en) | 1979-09-28 | 1981-03-31 | Philips Nv | OPTICAL FOCUS ERROR DETECTION SYSTEM. |
US4609813A (en) * | 1985-03-18 | 1986-09-02 | International Business Machines Corporation | Optical systems employing ovate light beams |
US4778984A (en) * | 1985-10-16 | 1988-10-18 | Canon Denshi Kabushiki Kaisha | Apparatus for detecting focus from astigmatism |
US4794585A (en) * | 1986-05-06 | 1988-12-27 | Lee Wai Hon | Optical head having a hologram lens and polarizers for use with magneto-optic medium |
EP0255305B1 (en) * | 1986-07-28 | 1993-01-07 | Sharp Kabushiki Kaisha | Focusing error detecting device and method of manufacturing the same |
JPH0760527B2 (en) * | 1986-09-17 | 1995-06-28 | パイオニア株式会社 | Optical pickup device |
NL8701749A (en) * | 1987-07-24 | 1989-02-16 | Philips Nv | DEVICE FOR SCANNING AN INFORMATION SHEET WITH OPTICAL RADIATION. |
JPH0814032B2 (en) | 1988-06-28 | 1996-02-14 | 日電アネルバ株式会社 | Dry etching equipment |
JPH07182687A (en) * | 1993-12-24 | 1995-07-21 | Sharp Corp | Optical pick-up |
JP3374573B2 (en) * | 1995-02-20 | 2003-02-04 | 松下電器産業株式会社 | Optical pickup and optical guide member |
JPH10500526A (en) * | 1995-03-15 | 1998-01-13 | フィリップス エレクトロニクス ネムローゼ フェンノートシャップ | Apparatus for optically scanning a recording medium |
JP3534363B2 (en) * | 1995-07-31 | 2004-06-07 | パイオニア株式会社 | Crystal lens and optical pickup optical system using the same |
-
1997
- 1997-05-28 JP JP9138779A patent/JPH10333025A/en active Pending
-
1998
- 1998-05-27 US US09/085,229 patent/US6445668B2/en not_active Expired - Fee Related
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
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US20080137495A1 (en) * | 2005-03-08 | 2008-06-12 | Mitsubishi Electric Corporation | Optical Device and Optical Disc Apparatus Utilizing the Optical Device |
US7839733B2 (en) * | 2005-03-08 | 2010-11-23 | Mitsubishi Electric Corporation | Optical device and optical disc apparatus utilizing the optical device |
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US6445668B2 (en) | 2002-09-03 |
JPH10333025A (en) | 1998-12-18 |
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